1. Field of the Invention
This invention relates to the field of remote, electronic identification using passive, RF Surface Acoustic Wave tagging devices. More particularly, this invention relates to a low-cost, miniaturized device with enhanced range that allows for a great number of codes to be placed on a Surface Acoustic Wave identification (SAW-ID) tag.
2. Description of the Prior Art
SAW devices are employed in identification systems, known as either SAW-ID tags or SAW wireless labels. The operating principle of these devices is well-known in the prior art: an incoming electromagnetic signal is received through an antenna by a transducer which launches a surface acoustic wave, which is encoded by structures on the surface of the device and propagated back to the transducer where the surface acoustic wave is reconverted into an electromagnetic signal and transmitted through the antenna.
These SAW-ID tag devices have numerous applications for logistics, material, personnel and vehicular identification and tracking, in both civilian and military environments, as well as identifying and tracking livestock, wildlife and other animals. Other possible or actual applications for SAW-ID tag devices include sorting luggage, mail or parcels, and preventing theft of books from a library or merchandise by tagging the item with a coded label and locating a receiver near an exit to detect unauthorized passage of a tagged article. See for example, U.S. Pat. No. 3,706,094 entitled "Electronic Surveillance System" issued to Cole and Vaughan on Dec. 12, 1972.
It is also known in the prior art that the outgoing signal can be coded by the placement of reflectors in an acoustic path, see for example a paper entitled "Programmable Reflectors for SAW-ID Tags", by L. Reindl and W. Ruile printed in the 1993 IEEE Ultrasonics Symposium. That paper notes that SAW-ID tags can replace bar codes and that Norway has implemented an RF vehicle identification system.
Those concerned with the development of SAW-ID tags have long recognized the need for a smaller and more versatile SAW-ID tag with enhanced range capability. Typically, SAW-ID tag devices are limited to a conveniently small size that will accommodate an array of codes on the device numbering in the millions but have a limited range of about 10-20 feet. Current size, range and cost limitations consequently restrict the applications for this technology. The aforementioned paper observed that while SAW-ID technology was in use or under development in several locations "the big breakthrough for SAW-ID tags has not yet occurred."
This invention provides a solution to the barriers of SAW-ID tags by addressing the current demand for a more versatile, low-cost, miniature SAW-ID technology, and the need for small labels with a much enhanced range, having a higher number of possible coded locations. This invention fulfills this need by utilizing pulse compression techniques with a significant processing gain for an enhanced range such that a one inch long SAW-ID tag could provide many as 1,000,000 code possibilities. Current SAW-ID tag devices operate in a range of about 10 feet, while the SAW-ID tag device of this invention offers a tenfold increased range. A key aspect of the ability to both miniaturize the SAW-ID tag and increase its effective range is the utilization of a nearby "chirp" transmitter that provides the necessary FM waveform (i.e. linear or nonlinear FM) to the input transducer through a built-in antenna on the SAW-ID tag.